1. Field of the Invention
This invention relates generally to the field of heaters for processing hydrocarbons and petrochemicals. More specifically this invention relates to electric resistance heaters for use in petrochemical and hydrocarbon conversion processes.
2. Description of the Prior Art
Most refining and petrochemical processes operate at elevated processing conditions. The feedstreams or intermediate streams entering these processes are often heated using electrical resistance heating. Electric resistance heating has advantages of safe and clean operations as well as compact and highly adaptable installations.
Electrical resistance heating uses a heater tube. The heater tube houses a heating element. These electric heaters usually employ a conductor in the form of a cold pin (sometimes referred to as terminal pin) which is used to electrically connect junction box terminal wiring to a wire resistance portion of an electric heater tube. The pin is termed "cold" because it does not generate heat like the resistance wire portion of the element. The cold pin extends the terminal into a portion of the heater tube across which the process stream flows and in which the portion of the resistance element passes.
In a typical tube construction the cold pin and resistance element are centered in a grounded metal tube and electrically insulated from that tube by a gas permeable insulating material. Heating elements of this type routinely use hollow tubing to provided the outer wall of the heating element to surround wire resistance elements. A non-electrically conductive packing surrounds a wire that is located in the tube. The packing fills the tube to center the wire and insulate the wire from the tube wall. Air permeates the packing to allow proper oxidation of the resistance wiring. A typical insulating material is magnesium oxide powder which has been compacted to a chalk like solid. Magnesium oxide, absorbs moisture at ambient conditions from the atmosphere. The electrical resistance of magnesium oxide drops proportionally with moisture content. The time period between manufacture of the heater tube and startup of the process equipment allows degradation of the electrical insulating capability of the magnesium oxide due to moisture ingress.
The cold pins extend into the tube through the packed material and connect with the wire. Moisture is absorbed at the terminal box end of the tube and penetrates into the element between the cold pin and the tube. Significant moisture penetration will not ordinarily proceed beyond the cold pin. The moisture ingress through an insulator such as a magnesium oxide element is believed to occur in a plug flow fashion. The cold pin area of the element is expected to saturate completely before the wire section of the element is reached. Therefore, the cold pin area is the most frequent location for ground faults due to moisture ingress. The cold pin is the most damaging area to have a short within the electrical element because there is minimal resistance between the power supply and the point of short.
Prior to initial electrical firing of the heater the insulation resistance is checked and low insulation values can prevent powering the heater. Applying design voltage to a heater with low resistance readings leads to electrical faults occurring through the wet insulation. Electrical faults cause permanent damage to elements. Faults can blow fuses, damage wiring, trip breakers, and cause damage to the process containing piping and flanges.
If detected before firing, the magnesium oxide, or other insulating powder, can be dried and insulation values restored by driving off moisture by subjecting it to high temperatures. The removal of moisture from the heater element insulation is an inconvenient and time consuming process.
Moreover, even where excess moisture in the packed insulation material is detected, problems develop when the moisture is not completely driven out before firing of the heater elements. In such cases a temperature gradient develops as the heater element section is powered. The gradient causes a concentration of moisture as vapor is driven to the point along the element where the dew point is reached. Moisture that concentrates in the cold pin area can produce a later detected ground fault.
Therefore, the presence of moisture interferes with the operation of heater tubes. Many tube manufactures seal heater tubes to prevent moisture ingress. Methods of sealing the heater tube or packing to prevent moisture ingress have not generally been successful. Moreover, even where the seal is successful, most wire heating elements must breath during operation or their life is reduced. It is important that the resistance element breathe in order to draw in oxygen. For example, in the case of nickel-chromium resistance wire the presence of oxygen at high temperature will form chromium oxide on its surface which will extend the life of the wire. Thus, achievement of maximum operational life mandates any seal be temporary and destroyed once the element is put into use. Once the seal is gone, any shutdowns can begin the moisture ingress again. Thus any prolonged down times for the heater can create the need to dry out the insulation again before firing.